1. Field of the Invention
The present invention relates to a communication device and a communication method immune to burst errors. More particularly, the present invention relates to a communication device (a transmitter, a receiver, and a transmitting/receiving device) which is immune to burst errors caused by fading or noise, a communication method performed by the communication device, a program for executing the communication method, and a computer-readable storage medium for storing the program.
2. Description of the Background Art
With recent improvements in LSI, an error correcting technology has been generally used in communications. As the error correcting technology, Golay codes, which are block correcting codes, are well known. For example, if the Golay (24, 12) code is used, 12 bit data is transmitted with 12-bit redundancy codes added thereto, whereby it is possible to correct errors up to three bits in total 24 bits (=one block). That is, in a case where errors occur in a random order, it is possible to correct errors even if a bit error rate is 0.125 (= 3/24). However, in a case where errors intensively occur in a certain period of time, in other words, in a case where so-called burst errors occur, error correction may be impossible. Now, in general, a plurality of blocks (N blocks) for which the above error correction is performed are transmitted for transmitting a lot of data (12×N bits). When a burst error occurs, errors occur intensively in a specific block of N blocks.
For example, a case where an error of four bits occurs in four blocks for which error correction of the Golay (24, 12) code is performed (a bit error rate=approximately 0.042) will be considered. In this case, if the above error occurs randomly in at least two of the four blocks, it is possible to correct errors because the number of error bits in one block does not exceed a correcting capability. However, if the burst error occurs intensively in any one block, it is impossible to perform error correction.
Thus, in order to improve a correcting capability with respect to the burst errors, a transmission technique using an interleaving method has been proposed. Hereinafter, the conventional transmission technique using the interleaving method will be described with reference to FIG. 14.
Part (a) of FIG. 14 is an illustration showing an exemplary method for transmitting four blocks, for which error correction of the Golay (24, 12) code is performed, after interleaving processing. In part (a) of FIG. 14, first to fourth blocks respectively contain 24 bits (12 bit data and 12 bit redundancy codes). In the conventional method, as indicated by dotted arrows in the drawing, a transmitting device sequentially transmits a first bit (1b) of the respective first to fourth blocks, and sequentially transmits a second bit (2b) of the respective first to fourth blocks. Similarly, the transmitting device sequentially transmits a third (3b) to 24th bit (24b) of the respective first to fourth blocks.
On the other hand, a receiving device cyclically distributes the received bits among the four blocks in the same manner as the transmitting device, and reconstructs the first to fourth blocks, respectively. Then, the receiving device performs an error correcting process for the respective reconstructed blocks.
In the above conventional transmission method, bits of the respective blocks are interleaved and transmitted, thereby handling a burst error occurring in intensive and sequential manners. For example, even in the above described case where a burst error of four bits occurs in intensive and sequential manners, only one bit error occurs on a block-by-block basis in the conventional transmission method, whereby error correction can be performed for all the blocks (part (b) of FIG. 14).
However, in a case where the error rate is further deteriorated and a burst length (duration time) of the burst error is increased, a correcting capability of the conventional method quickly reaches its limit. For example, in a case where a burst error of 13 bits occurs intensively in four blocks (a bit error rate=approximately 0.135), at least four bit errors occur in any one block. As a result, it is impossible to perform error correction for the block where four bit errors occurred (part (c) of FIG. 14). That is, even the conventional method using interleaving processing has a problem that error correction cannot be performed for all the blocks if a burst error whose error rate exceeds a random error correcting capability occurs in any block.
In order to address the above problem, in power line carrier communications, for example, an error correcting technique has been proposed (Japanese Patent Laid-Open Publication No. H11-266190) utilizing a fact that a burst error, which is noise, occurs cyclically in synchronization with commercial power (see FIG. 15). A device disclosed in the above gazette includes, as shown in FIG. 16, a transmitter 102 having a power synchronous signal generator 106 for generating a power synchronous signal 105 from commercial power passing through a lamp line 104, a timing signal generating section 110 for generating a timing signal, which is obtained by dividing the power synchronous signal 105 into a plurality of signals, and a control section 111 for transmitting, based on the timing signal, the same information frame (packet) to the lamp line 104 a plurality of different times in order to prevent noise synchronized with the commercial power passing through the lamp line 104 from falling on the same location of the packet. Also, the device disclosed in the above gazette includes a receiver 103 having a storing section 112 for storing a plurality of information frames (packet) received via the lamp line 104, a determining section 113 for determining that an information frame (packet) is a correct signal from among the plurality of information frames, which are the same information frame transmitted the plurality of times, if the information frame includes the least bit error, and a control section 111 for controlling the storing section 112 and the determining section 113.
However, the device disclosed in the above gazette has the following problems.
First, the above device has to include the power synchronous signal generator 106 and the timing signal generating section 110 in order to detect a zero cross of power. Thus, the above two components make it difficult to reduce the size and cost of the device. Also, the above method of determining a cycle of a burst error by detecting a zero cross of the power is applied only to the power line carrier communications.
Second, there arises a problem by transmitting a plurality of frames (packets). In order to perform distributed control for a plurality of terminals, it is necessary to equally enable the respective terminals to perform transmission. Thus, it is preferable to lengthen a transmission time, to some extent, of each transmission, and cause a terminal to pause for some time after transmission. The Japanese Radio Law requires one transmission time and a pause time to be 200 ms, at the maximum, and 40 ms, respectively. Here, a half-cycle of 50 Hz takes 10 ms. Thus, in order to comply with the Japanese Radio Law, a pause time (40 ms) which is quadruple of the half cycle is required for each frame (packet) transmission. Also, in order to perform communications successfully even if a duration time of a burst error (noise) is increased, it is not sufficient to transmit a plurality of frames (packets) three times as exemplified in the above document. As a result, overhead caused by the increased pause time becomes so great that transmission efficiency is reduced and delay times are increased. Especially, in a case where a method of transmitting a plurality of frames (packets) is applied to device control, the increased delay times results in slow response. Thus, a system based on the method of transmitting a plurality of frames (packets) has no practical use.